plex or Jellyfin?

It depends what you use it for.

If you're watching your own content within your home then Jellyfin is better. It's free, open source and private. Your Jellyfin instance is yours and secure, and entirely under your control.

Plex's differences are mostly behind it's plex pass pay wall, and you sacrifice privacy using their platform. The key difference is really offline and remote viewing of content which is easier and slicker with plex (but doable with jellyfin), and the plex App maybe available a few more devices. There are also some credits and ad skipping features. That's about it - I struggle to see the benefit in plex. The only other thing I can think of is some people prefer the interface?

I used to use Plex and got annoyed when I couldn't view my content, which I host locally, because their login servers were down. Made me realise why did I need them so I researched a bit and switched to Jellyfin.